Wireless networks have experienced increased development in the past decade. One of the most rapidly developing areas is mobile ad hoc networks. Physically, a mobile ad hoc network includes a number of geographically-distributed, potentially mobile nodes wirelessly connected by one or more radio frequency channels. Compared with other type of networks, such as cellular networks or satellite networks, the most distinctive feature of mobile ad hoc networks is the lack of any fixed infrastructure. The network is formed of mobile nodes only, and a network is created on the fly as the nodes transmit to or receive from other nodes. The network does not in general depend on a particular node and dynamically adjusts as some nodes join or others leave the network.
In a hostile environment where a fixed communication infrastructure is unreliable or unavailable, such as in a battle field or in a natural disaster area struck by earthquake or hurricane, an ad hoc network can be quickly deployed and provide much needed communications. While the military is still a major driving force behind the development of these networks, ad hoc networks are quickly finding new applications in civilian or commercial areas. Ad hoc networks will allow people to exchange data in the field or in a class room without using any network structure except the one they create by simply turning on their computers or PDAs.
As wireless communication increasingly permeates everyday life, new applications for mobile ad hoc networks will continue to emerge and become an important part of the communication structure. Mobile ad hoc networks pose serious challenges to the designers. Due to the lack of a fixed infrastructure, nodes must self-organize and reconfigure as they move, join or leave the network. All nodes could potentially be functionally identical and there may not be any natural hierarchy or central controller in the network. Many network-controlling functions are distributed among the nodes. Nodes are often powered by batteries and have limited communication and computation capabilities. The bandwidth of the system is usually limited. The distance between two nodes often exceeds the radio transmission range, and a transmission has to be relayed by other nodes before reaching its destination. Consequently, a network has a multihop topology, and this topology changes as the nodes move around.
The Mobile Ad-Hoc Networks (MANET) working group of the Internet Engineering Task Force (IETF) has been actively evaluating and standardizing routing, including multicasting, protocols. Because the network topology changes arbitrarily as the nodes move, information is subject to becoming obsolete, and different nodes often have different views of the network, both in time (information may be outdated at some nodes but current at others) and in space (a node may only know the network topology in its neighborhood usually not far away from itself).
A routing protocol may need to adapt to frequent topology changes and with less accurate information. Because of these unique requirements, routing in these networks is very different from others. Gathering fresh information about the entire network is often costly and impractical. Many routing protocols are reactive (on-demand) protocols: they collect routing information only when necessary and to destinations they need routes to, and do not generally maintain unused routes after some period of time. This way the routing overhead is greatly reduced compared to pro-active protocols which maintain routes to all destinations at all times. It is important for a protocol to be adaptive. Ad hoc On-demand Distance Vector (AODV), Dynamic Source Routing (DSR) and Temporally Ordered Routing Algorithm (TORA) are representative of on-demand routing protocols presented at the MANET working group.
Examples of other various routing protocols include Destination-Sequenced Distance Vector (DSDV) routing which is disclosed in U.S. Pat. No. 5,412,654 to Perkins, and Zone Routing Protocol (ZRP) which is disclosed in U.S. Pat. No. 6,304,556 to Haas. ZRP is a hybrid protocol using both proactive and reactive approaches based upon distance from a source node.
These conventional routing protocols use a best effort approach in selecting a route from the source node to the destination node. Typically, the number of hops is the main criteria (metric) in such a best effort approach. In other words, the route with the least amount of hops is selected as the transmission route.
In a dynamic ad-hoc network system, path stability is important because links in the path are likely to break. Current ad-hoc mechanisms switch entire paths upon route failures. A broken path results in lost time to switch or re-establish new paths. Maintenance of multiple paths improves the path switching reaction time, but it is done at the complete path level and there is still time lost in propagating the new routing information to all nodes in the path.
Using the routing protocols to address the sudden lost of a data path is a typical approach. Most of the existing approaches are in the domain of the routing level, not the data-path level. Typical ad-hoc network routing protocols in some way address the routing maintenance issue. When a route is broken, it will be detected by a maintenance mechanism, and a new route will be created. Some routing protocols will try to provide routing updates in a periodical fashion, regardless if the routes are being used or not. Unfortunately, routing updates use a lot of the bandwidth resource and there is a significant latency for the route convergence.
Some routing protocols will create or re-discover a route only if it is on-demand. This type of protocol tends to have higher latency in the initial route setup but it is more bandwidth efficient. It may also provide a route maintenance mechanism, for local repair of an active used route. Predictive routing is a higher level of intelligence which schedules and directs the lower layer routing protocols to actively prepare routes by using historical and statistical analysis. It is in fact a high level tuning of the lower layer routing protocols.
Multiple-path approaches exist such as Ad hoc On-demand Multipath Distance Vector (AOMDV) Routing, Ad hoc On-Demand Distance Vector Multipath (AODVM), Node Disjoint Multipath Routing (NDMR), Multipath DSR (MP-DSR), and Split Multipath Routing (SMR). These protocols use different routing mechanisms to setup the multiple paths. Stability is achieved by using one path until it is broken, then shifting to another one. Detecting the break down of a path is required, but a new path is immediately available. All paths may be used and traffic may be shared for load balancing. Duplicate data may be sent on some paths. Bandwidth is wasted but better fault tolerance is achieved with a reduced reaction time.
The link-breaking reaction time is still not minimal. There is a need for an approach that reduces link outage time and enhances the overall effectiveness of multipath routing.